Thermal barrier coatings are thin ceramic layers that are used to insulate air-cooled metallic components from high temperature gases, for example in gas turbine or other heat engines. Such coatings are useful in protecting and extending the service life of metallic components exposed to high temperatures, such as jet engine turbine blades and combustor components. Thermal barrier coatings composed of yttria-stabilized zirconia are known, wherein the yttria typically makes up seven to nine weight percent (or four to five molar percent) of the total composition. These coatings are typically applied using plasma spraying or physical vapor deposition techniques in which melted ceramic particles or vaporized ceramic clouds are deposited onto the surface of the component that is to be protected. Thermal barrier coatings are somewhat porous with overall porosities generally in the range of 5 to 20%. This porosity serves to reduce the coating's effective thermal conductivity below the intrinsic conductivity of the dense ceramic, as well as to improve the coating's strain tolerance. However, the coating conductivity will increase as the porosity decreases in high temperature service due to ceramic sintering.
In a jet engine, higher operating temperatures generally lead to greater efficiency. However, higher temperatures also cause more problems such as higher stresses, increased materials phase instability and thermal oxidation, leading to premature failure of components. A ceramic coating with lower thermal conductivity and improved high temperature stability would allow higher operating temperatures while preserving operating life of the coated component. Accordingly there is a need for thermal barrier coatings with a lower conductivity and better sintering resistance than prior art coatings. Such a coating ideally would retain low conductivity after many hours of high temperature service. A laser test, recently developed by the current inventors has allowed simultaneous testing of durability, conductivity, and conductivity increase due to sintering under turbine-level high heat flux conditions. See, e.g., Dongming Zhu and Robert A. Miller, “Thermal Conductivity and Elastic Modulus Evolution of Thermal Barrier Coatings under High Heat Flux Conditions,” Journal of Thermal Spray Technology Vol. 9(2) June 2000 pp. 175–180, and “Thermophysical and Thermomechanical Properties of Thermal Barrier Coating systems,” Ceramic Engineering and Science Proceedings, Vol. 21, 2000 pp. 623–633, both of which are incorporated herein by reference. The thermal barrier coating advances described in this application have had the benefit of this new test approach.